1. Field of the Invention
The present invention relates to extrusion engineering for thermoplastically deformable plastics, in particular to the control of the height of bulge in the glazing roll gap for supercalender devices for extrusion of plastic sheets for plates or films that are smoothed on both sides. In particular, the present invention relates to apparatus and methods for the extrusion of thermoplastic sheets, plates, and films.
2. Discussion of the Background
Extrusion systems for thermoplastic materials are known. Usually supercalenders (roll-mills) are employed for smoothing the two surfaces of plastic sheets for plates or films. The purpose of the rolls in this case is for shaping and cooling the molten plastic. The rolls take the plastic exiting vertically from a fishtail die in the molten state into the glazing roll gap between two driven inlet rolls. Usually the plastic sheet is taken off by one of the inlet rolls and diverted into the horizontal direction by means of a second roll, which in turn presses against the inlet roll. The plastic sheet attains a defined thickness and a smooth surface due to its being pressed through the glazing roll gap between the inlet rolls.
Factors that affect the surface condition of plastic sheets for plates and films smoothed on both sides are the composition of the polymer, the temperature and the viscosity of the molten polymer stream, the temperature and the surface condition of the rolls, the size of the glazing roll gap and the roll speed.
A uniform passage of the plastic through the glazing roll gap in this case is exceptionally important for obtaining a satisfactory surface of the extruded material. In this case, to prevent surface malformations, the height of the so-called bulge (or flashback) is very important. The bulge forms in the glazing roll gap between the inlet rolls due to the backwash of the polymer melt. Fluctuations due to local differences in mass flow in the polymer melt exiting the extrusion nozzle lead to differing melt accelerations at the surface of the inlet roll, so that the height of the bulge also changes.
The height of the bulge should only fluctuate within the smallest possible tolerances. If it is too low, the contact of the extrudate with the roll 1 can be lost. This results in a poor extrudate surface or can lead to sticking to the roll 2 due to the highly changed cooling conditions. The so-called wrapping then leads to the collapse of the process and to a system shutdown lasting several hours.
An excessive bulge leads to malformations of the extrudate surface due to the so-called bulge roll. In this case, due to contact with the inlet extrudate stream, the bulge is contracted coil-like from above and rolled up to a certain extent. This leads to formation of streaks running transverse to the surface of the extrudate. In an extreme case, the stagnation bulge can also reach the lip of the nozzle and foul it, which in turn leads to malformation of the extrudate surface. The nozzle can only be cleaned when the system is shut down.
Several methods for controlling the height of the bulge are known. In practice, it is commonplace to implement a manual adjustment of the height of the bulge by measuring the thickness of the cooled plastic sheet and performing the adjustment by a change in the speed of the roll. The disadvantage of this method is that the intervention takes place manually and is thus exposed to subjective influences. Due to the relatively delayed measurement of thickness, there is moreover a comparatively high reject rate until the adjustments are within the desired tolerance.
In addition, direct measurements of the height of the bulge by means of mechanical sensors or by the laser triangulation method are known. The mechanical method is very imprecise, whereas the optical method has the disadvantage that it is fraught with error due to the high temperatures occurring in the vicinity of the stagnation bulge and due to the resultant fluctuation in air density.
An automatic control operating by means of an adjusting dial is described in DE 40 33 661 C2. The contact pressure is measured continuously on one of the inlet rolls as a measure for the thickness of the material sheet and is controlled by means of the roll speed. However, since the contact pressure depends not only on the thickness of the material sheet, but also on the viscosity and the temperature of the plastic stream, this control is also unsatisfactory.
From G 92 12 406.2, the control of the height of bulge for the extrusion of transparent films with a thickness of less than 5 mm is known. The measurement of the orientation birefringence of the extruded film was used as a measure for the height of the bulge and was controlled by the change of the setting of the mass flow of the extrusion nozzle.
Greek Patent No. 85.1420 describes a device to control the height of the bulge in the production of a plastic sheet smoothed on both sides. The size of the bulge is held constant by using as information the surface temperature of the sheet after its outlet from the roll gap. In case of deviations from the desired temperature associated with the state of continuous operation, the rate of extrusion of mold material is increased or decreased. The disadvantage of this method is that it is too inaccurate.
Devices that seek to control the height of the bulge by means of measuring the width of the extruded material are not known. Thus, there remains a need for a method and apparatus for controlling the height of the bulge of thermoplastic in the glazing roll gap of a supercalender in the extrusion of thermoplastic plates, films, or sheets.